Interaction between ionic liquid cation and water: infrared predissociation study of [bmim](+)·(H2O)n clusters

The infrared predissociation spectra of [bmim](+)·(H2O)n, n = 1-8, in the 2800-3800 cm(-1) region are presented and analyzed with the help of electronic structure calculations. The results show that the water molecules solvate [bmim](+) by predominately interacting with the imidazolium C2-H moiety for the small n = 1 and 2 clusters. This is characterized by a redshifted and relatively intense C2-H stretch. For n≥ 4 clusters, hydrogen-bond interactions between the water molecules drive the formation of ring isomers which interact on top of the imidazolium ring without any direct interaction with the C2-H. The water arrangement in [bmim](+)·(H2O)n is similar to the low energy isomers of neutral water clusters up to the n = 6 cluster. This is not the case for the n = 8 cluster, which has the imidazolium ring disrupting the otherwise preferred cubic water structure. The evolution of the solvation network around [bmim](+) illustrates the competing [bmim](+)-water and water-water interactions.